The present application relates to patent application U.S. Ser. No. 09/264,510, filed concurrently with this application, which is hereby incorporated by reference.
The present invention relates to wide range oxygen sensors and especially relates to a simplified wide range oxygen sensor design.
Oxygen sensors are used in a variety of applications which require qualitative and quantitative analysis of gases. In automotive applications, the direct relationship between oxygen concentration in the exhaust gas and air to the fuel ratio of the fuel mixture supplied to the engine allows the oxygen sensor to provide oxygen concentration measurements for determination of optimum combustion conditions, maximization of fuel economy, and the management of exhaust emissions.
A conventional stoichiometric oxygen sensor typically consists of an ionically conductive solid electrolyte material, a porous electrode on the sensor""s exterior exposed to the exhaust gases with a porous protective overcoat, and a porous electrode on the sensor""s interior surface exposed to a known oxygen partial pressure. Sensors typically used in automotive applications use a yttria stabilized zirconia based electrochemical galvanic cell with porous platinum electrodes, operating in potentiometric mode, to detect the relative amounts of oxygen present in an automobile engine""s exhaust. When opposite surfaces of this galvanic cell are exposed to different oxygen partial pressures, an electromotive force is developed between the electrodes on the opposite surfaces of the zirconia wall, according to the Nernst equation:   E  =            (              RT                  4          ⁢          F                    )        ⁢          ln      ⁡              (                              P                          O              2                        ref                                P                          O              2                                      )            
where:
E=electromotive force
R=universal gas constant
F=Faraday constant
T=absolute temperature of the gas
PO2ref=oxygen partial pressure of the reference gas
PO2=oxygen partial pressure of the exhaust gas
Due to the large difference in oxygen partial pressure between fuelrich and fuellean exhaust conditions, the electromotive force changes sharply at the stoichiometric point, giving rise to the characteristic switching behavior of these sensors. Consequently, these potentiometric oxygen sensors indicate qualitatively whether the engine is operating fuelrich of fuellean, without quantifying the actual air to fuel ratio of the exhaust mixture.
Increased demand for improved fuel economy and emissions control has necessitated the development of oxygen sensors capable of quantifying the exhaust oxygen partial pressure over a wide range of air fuel mixtures in both fuel-rich and fuel-lean conditions. As is taught by U.S. Pat. No. 4,863,584 to Kojima et al., U.S. Pat. No. 4,839,018 to Yamada et al., U.S. Pat. No. 4,570,479 to Sakurai et al., and U.S. Pat. No. 4,272,329 to Hetrick et al., an oxygen sensor which operates in a diffusion limited current mode produces a proportional output which provides a sufficient resolution to determine the air-to-fuel ratio under fuel-rich or fuel-lean conditions. Generally, diffusion limited current oxygen sensors have a pumping cell and a reference cell with a known internal or external oxygen partial pressure reference. A constant electromotive force, typically corresponding to the stoichiometric electromotive force, is maintained across the reference cell by pumping oxygen through the pumping cell. The magnitude and polarity of the resulting diffusion limited current is indicative of the exhaust oxygen partial pressure and, therefore, a measure of air-to-fuel ratio.
As is taught by U.S. Pat. No. 4,450,065, wide range oxygen sensors commonly employ an aperture with a cross-sectional area to length ratio sufficiently small to limit exhaust gas diffusion. In this sensor, a gap between the pumping and reference cells forms such an aperture and limits diffusion of exhaust gas to a common environment between the two cells. This common environment, or diffusion chamber is required in an aperture construction for adequate mixing of the diffused exhaust gas; however, it tends to slow the frequency response of the sensor operation. Additionally, although the two electrodes adjacent to the diffusion chamber can be shorted together to eliminate one lead, four separate electrodes are required in this construction.
Commonly assigned U.S. Pat. No. 5,360,528 to Oh et al., teaches a wide range oxygen sensor having improved mass production capabilities. This wide range oxygen sensor employs a porous layer, formed by plasma spray deposition, to limit oxygen diffusion in lieu of the diffusion limiting aperture. Referring to FIG. 1, this wide range oxygen sensor 16 has a planar structure with a single solid electrolyte layer 6 shared by electrochemical storage (4/10/6/8), pumping (2/12/6/14) and reference (10/6/12) cells. The electrochemical pumping cell has a diffusion layer 2 formed from a porous ceramic to permit diffusion of oxygen molecules through this layer.
Although the wide range oxygen sensor of Oh et al. eliminates the need for a mixing chamber between the pumping and reference cells and improves mass production capabilities thereof, there still exists a need to further improve the processing and assembly of wide range oxygen sensors in mass production.
The present invention relates to methods for producing oxygen sensors. In one embodiment the method comprises: preparing a planar first electrolyte layer having a portion of first electrolyte tape and a portion of first substrate material tape wherein at least one edge of said first electrolyte tape abuts said first substrate material tape; laminating said first electrolyte layer; depositing an outer electrode on said laminated first electrolyte layer, depositing an inner electrode on a solid electrolyte layer, depositing a reference electrode on a reference electrode layer; depositing a heater on a heater side of a heater layer, stacking said solid electrolyte layer and said first electrolyte layer such that said inner electrode is disposed therebetween, in contact with both said solid electrolyte and said first electrolyte, disposing a first side of said reference electrode layer adjacent to said solid electrolyte layer such that said reference electrode is in contact with said solid electrolyte, disposing said heater side of said heater layer on a second side of said reference electrode layer, and laminating said first electrolyte layer, said solid electrolyte layer, said reference electrode layer and said heater layer together.
In another embodiment the method comprises: preparing a planar layer having a first portion of first material tape and a second portion of second material tape, wherein at least one edge of the first material tape abuts at least another edge of the second material tape; compressing said first planar layer to form a single planar piece, wherein the first and second materials are coplanar and have an interface between said one edge and said another edge; depositing a conductive material on a first side of said single planar piece across the interface; stacking said first side and/or a second side of said single planar piece with at least one additional planar piece; laminating said stacked single planar piece and said at least one additional planar piece; and sintering the laminated and stacked single planar piece and at least one additional piece.
These and other objects, features and advantages of the present invention will be apparent from the following brief description of the drawings, detailed description, and appended claims and drawings.